Such a method and such an apparatus are known from US 2005/0117696 A1. According to the known method a panoramic image is taken from the dentition of a patient. The panoramic image is further used for selecting a region of interest. Subsequently, a tomographic imaging process is performed for obtaining a three-dimensional image of the region of interest. For both processes the patient is kept stationary at the same place using a single head support.
One disadvantage of the known method and the known apparatus is that the apparatus is primarily arranged for computer tomography based on two-dimensional projection images taken by digital flat panel detectors. The flat panel detectors can be detectors with scintillators that transform the incident X-ray radiation into light in the optical wavelength range. The light generated by the scintillators is then detected by optical detectors. As set forth in detail in US 2008/0063139 A1, this type of detectors can hardly be used for panoramic imaging since these detectors are not fast enough for continuous exposure. US 2008/0063139 A1 therefore proposes to use CMOS detectors based on CdTe for panoramic imaging. However, these detectors are quite expensive. In addition, these detectors must be read out at a high frame rate, so that the read-out electronic must be relatively fast. Furthermore large storage volumes are required for storing the vast amount of data.
U.S. Pat. No. 5,921,927 A discloses a further method and apparatus. In a first step of the known method an imprint of the mouth of the patient is taken. Afterwards, the imprint is attached to the radiographic apparatus through a specific holder. The imprint and the holder form together a bite unit. The holder is then moved manually by the clinical operator, to bring it in the desired position. According to the known method, an optical adjustment device is used to align the bite unit of the apparatus such that a local tomographic image can be taken from a particular region of dentition of the patient. The optical adjustment device is provided with a laser whose beam can be used to mark the region of tomographic imaging on the imprint in the bite unit. For adjusting the patient, the operator moves the bite unit until the point of interest on the imprint matches with the mark of the laser beam on the imprint.
U.S. Pat. No. 6,424,694 B1 discloses a bite support that is translated and oriented by an external off-line alignment accessory device, so that the center of the volume of interest falls under the reference marking of a transparent plexiglas template. The device is then repositioned on the system to perform the tomographic examination.
In both cases, the operator has to modify position and orientation of the bite support manually in order to adapt it to the patient and to the desired volume of interest. This procedure is time consuming and it may introduce substantial errors in patient positioning.
Local tomographic analysis is a widely used examination in the field of dental surgery, in particular in the field of implantology. Conventional tomography, tomosynthesis, and local cone beam computerized tomography (=CB-CT) are well known techniques, which generate a volumetric reconstruction of the analyzed anatomical districts, delivering a low radiation dose in the patient tissues.
To achieve the maximum efficiency, patient positioning is critical. In particular the patient's position with respect to the radiographic apparatus has to be set such that the center of the volume of interest coincides with the center of the volume which will be reconstructed. This problem is common to conventional tomography, tomosynthesis and CB-CT.
Conventional tomography and tomosynthesis are limited angle of view tomographic methods that generate a set of parallel slices orthogonal to main direction of projection, where resolution is minimum along the direction orthogonal to the slices, and maximum along the slices. Therefore, conventional tomography and tomosynthesis also require that the patient is adequately oriented in the three-dimensional space, with respect to the radiographic apparatus. Incorrect orientation of the acquired volume may introduce blurring of the interesting anatomical structures, making structure identification difficult and measurements inaccurate.
For instance, volume orientation is critical when measurements have to be taken along a preferential direction. This is common to different dental applications like implantology, where the knowledge of the lower or upper jaw cross-section and of the nerve canal position is fundamental to plan an accurate, risk free, implant. In this situation, the slices acquired with conventional tomography or tomosynthesis should be orthogonal to the lower or upper jaw profile, to allow the maximum resolution of the anatomical structures and accurate measurements of the dimension of the anatomical structures of interest.
For CB-CT the orientation of the volume is also important, especially if the reconstructed volume has an anisotropic shape, or if a limited angle of view acquisition scheme is adopted.
U.S. Pat. No. 5,425,065 A discloses a method and an apparatus for taking panoramic images of the dentition of a patient. The panoramic radiography is based on the theory of orthopantomography, which has been established for a long time. The basic principle of orthopantomography is a coordinated movement of the X-ray source and detector pair, which allows focusing on the structures lying on a predefined surface, blurring out at the same time structures outside the focal plane. This approach is also used in panoramic radiography to focus on the patient dental arch, blurring all the other anatomical structures of the skull.
BRAUN, S. et al., The shape of the human dental arch, The Angle Orthodontist, Vol. 68, No. 1, p. 29-36, 1998 contains data on a template of the human dental arch.
SAHIWAL, I. G. et al., Radiographic identification of nonthreaded encloseous dental implants, J. Prosthet. Dent. 87, 552-562 (2002) and LEHMANN, T. et al. IDEFIX—Identification of dental fixtures in intraoral X-rays, Proc. SPIE 2710, p. 584-595 (1996) disclose methods for the identification of fixtures within the dental arch such as dental implants.